1. Field of the Invention
The present invention relates to a disc recording unit and more particularly to a disc drive that reads or writes information to or from a recording disc such as a CD or DVD. Still more particularly, the present invention relates to a head driver that moves the recording head over a recording disc.
2. Description of the Background Art
Recently, there is much effort being made to develop an optical disc drive as a disc recording drive. Among other things, an optical disc drive is required to write data at a high speed. A driver, which radially moves an optical head for recoding information signals on an optical disc, is also required to feed the head precisely. An optical head driver with the rack-and-pinion mechanism is proposed as an optical head driver satisfying this requirement. FIG. 11 is a front view showing the conceptual configuration of the driver. A chassis 2, has a spindle motor 4 thereon that rotates an optical disc not shown. Also provided on the chassis 2 is a pair of guide rails 3, 3 on which an optical head 1 is provided such that it can slidingly move back and forth over the optical disc in the radial direction. The optical head 1 has a rack 6 secured by a screw 7 on one side along which the optical head 1 slidingly moves. In the area on the chassis 2 opposed to a rack teeth 6A of the rack 6, a pinion 11, which engages the rack tooth 6A, a drive motor 10, and an intermediate gear 12 that rotates the pinion 11 are provided. In this configuration, an output gear 13 on the rotational output axis of the drive motor 10 is rotated. The output gear 13 rotates the pinion 11 via the intermediate gear 12. Then, because the pinion 11 and the rack 6 are engaged, the rack 6, that is, the optical head 1 slidingly moves along the rails.
This type of optical head driver has a structure in which the rack 6 is secured on the optical head 1 by engaging the boss on the rack 6 in the hole, not shown, on the optical head 1 for positioning. This implies that a size error, if introduced during hole and/or boss manufacturing, would sometimes result in the rack 6 being misaligned with the sliding direction of the optical head 1. When the optical head 1 slidingly moves along the rails in this state, the spacing between the rack 6 and the pinion 11 varies. When they are too close, the pinion 11 cannot rotate; when they are widely apart, a backlash occurs between them. This condition sometimes results in an abnormal sound being generated, the feeding load of the optical head being varied, and the optical head positioning precision being degraded, thus decreasing the reliability of the optical disc drive.
Japanese Patent Laid-Open Publication No. 2001-195848 and Japanese Patent Laid-Open Publication No. 2001-23322 have proposed technologies for proper engagement between the rack and the pinion. According to the technology disclosed in the former invention, the pinion is pressed against the rack by the spring force. On the other hand, the cylinder surface of the pinion engages the flat surface on the rack. This engagement adjusts the preload applied to the rack by the pinion through the spring force. This structure ensures the proper engagement between the pinion and the rack even if the rack installation condition varies. According to the technology disclosed in the latter invention, a motor installation board, on which the motor, intermediate gear, and pinion are installed, is pressed against the rack by the spring force. This structure enables the optical head to move back and forth under a fixed rotational load regardless of the rotational direction of the intermediate gear.
The above publications both employ a technology for pressing the pinion against the rack by the spring force to ensure proper engagement between the pinion and the rack. Therefore, even if the installation direction of the rack is misaligned with the sliding direction of the optical head, they are engaged properly. As a result, the technology solves the problems described above; that is, it prevents the pinion from not rotating properly and a backlash from being generated between the pinion and the rack. However, the spring force is always applied between the pinion and the rack. This generates a problem in the smooth movement of the optical head when it moves at a high speed.
That is, the high-speed movement of the optical head requires the optical head to be driven by a high motor torque generated by an efficient gear ratio. However, an attempt to do so exerts a force on the mating surface between the pinion and the rack, and the force prevents the pinion from properly engaging with the rack. This sometimes results in the pinion being disengaged from the rack. The technology disclosed in the above publications presses the pinion against the rack by the spring force. This is efficient for preventing the disengagement between the pinion and the rack. However, this technology requires the spring force strong enough for engaging the pinion with the rack even if the optical head is moved by the maximum output of the motor. The technology disclosed in the former publication adjusts the force applied between the gear tooth of the pinion and those of the rack. However, even in this case, the spring force ensuring the engagement between the pinion and the rack is required. On the other hand, to improve the recording characteristics of the optical head, it is required that the optical head be moved smoothly. To move the head smoothly, an excess force should not be applied to the gear tooth surface of the pinion and that of the rack. However, according to the technology disclosed in the publications described above, the spring force increases the force of engagement between the pinion and the rack. This adds an excess force to the gear tooth surface of the pinion and that of the rack. This presents a problem in high-speed, smooth movement of the optical head.